A hydrogel is a hydrated cross-linked polymeric system that contains water in an equilibrium state. Hydrogels typically are oxygen permeable and biocompatible, making them a preferred material for producing biomedical devices and in particular contact or intraocular lenses.
Conventional hydrogels are prepared from monomeric mixtures predominantly containing hydrophilic monomers, such as 2-hydroxyethyl methacrylate (HEMA) or N-vinyl pyrrolidone (NVP). U.S. Pat. Nos. 4,495,313; 4,889,664 and 5,039,459 disclose the formation of conventional hydrogels. Oxygen permeability of these conventional hydrogel materials relates to the water content of the materials, and is typically below 20-30 barrers. For contact lenses made of the conventional hydrogel materials, that level of oxygen permeability is suitable for short-term wear of the contact lenses; however, that level of oxygen permeability may be insufficient to maintain a healthy cornea during long-term wear of contact lenses (e.g. 30 days without removal). Therefore, efforts have been made and continue to be made to increase the oxygen permeability of conventional hydrogels.
One known way to increase the oxygen permeability of the hydrogels is to add silicone-containing monomers to the hydrogel formulations, thereby making silicone hydrogels. Silicone-containing polymers generally have higher oxygen permeabilities than conventional hydrogels. Silicone hydrogels have been prepared by polymerizing mixtures containing at least one silicone-containing monomer and at least one hydrophilic monomer. Either the silicone-containing monomer or the hydrophilic monomer may function as a crosslinking agent (a crosslinking agent is a monomer having multiple polymerizable functionalities) or a separate crosslinking agent may be employed. The formation of silicone hydrogels has been disclosed in U.S. Pat. Nos. 4,954,587; 5,010,141; 5,079,319; 5,115,056; 5,260,000; 5,336,797; 5,358,995; 5,387,632; 5,451,617; and 5,486,579; and WO 96/31792.
U.S. Pat. No. 3,808,178 discloses the formation of copolymers of small silicone-containing monomers and various hydrophilic monomers.
U.S. Pat. No. 5,034,461 describes silicone hydrogels prepared from various combinations of silicone-polyurethane macromers and hydrophilic monomers such as HEMA or N,N-dimethyacrylamide (DMA). The addition of methacryloxypropyltris(trimethylsiloxy)silane (TRIS) reduced the modulus of such hydrogels, but in many examples the modulus was still higher than may be desired.
U.S. Pat. Nos. 5,358,995 and 5,387,632 describe hydrogels made from various combinations of silicone macromers, TRIS, n-vinyl pyrrolidone (NVP) and DMA. Replacing a substantial portion of the silicone macromer with TRIS reduced the modulus of the resulting hydrogels. Two publications from the same author, "The Role of Bulky Polysiloxanylalkyl Methacrylates in Polyurethane-Polysiloxane Hydrogels", J. Appl. Poly. Sci., Vol. 60, 1193-1199 (1996), and "The Role of Bulky Polysiloxanylalkyl Methacrylates in Oxygen-Permeable Hydrogel Materials", J. Appl. Poly. Sci., Vol. 56, 317-324 (1995) also describe experimental results indicating that the modulus of hydrogels made from reaction mixtures of silicone-macromers and hydrophilic monomers such as DMA decreases with added TRIS.
The use of methacryloxypropylbis(trimethylsiloxy)methylsilane (MBM) to make hard contact lenses was described in WO 9110155 and in JP 61123609.
When relatively high levels of bulky silicone-containing monomers such as TRIS are incorporated into the hydrogels made from silicone-containing macromers and hydrophilic monomers, the elasticity, or the speed at which the polymer returns to its original shape after stress can be reduced to an extent that is unacceptable to the contact lens wearer.
There still remains a need in the art for silicone hydrogels which are soft enough to make soft contact lenses, which possess high oxygen permeability, suitable water content, and sufficient elasticity, and are comfortable to the contact lens wearer.